The present invention relates generally to a prosthesis, and more particularly to a modular prosthesis having a stem component with a counterbored cavity defined therein and associated method.
During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure may involve the use of a prosthesis which is implanted into one of the patient""s bones. In the case of a hip replacement procedure, a femoral prosthesis is implanted into the patient""s thigh bone or femur. The femoral prosthesis is typically constructed as a one-piece structure having an upper portion which includes a spherically-shaped head which bears against the patient""s pelvis or acetabulum, along with an elongated intramedullary stem which is utilized to secure the femoral component to the patient""s femur. In order to secure the prosthesis to the patient""s femur, the medullary canal of the patient""s femur is first surgically prepared (e.g. reamed and/or broached) such that the intramedullary stem of the femoral prosthesis may be subsequently implanted therein. The femoral prosthesis may be press fit into the medullary canal or, in the alternative, bone cement may be utilized to secure the femoral prosthesis within the medullary canal.
During performance of a joint replacement procedure, it is generally necessary to provide the surgeon with a certain degree of flexibility in the selection of a prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. For example, in the case of a femoral prosthesis, the patient""s femur may be relatively long or relatively short thereby requiring use of a femoral prosthesis which includes a stem that is relatively long or short, respectively. Moreover, in certain cases, such as when use of a relatively long stem length is required, the stem must also be bowed in order to conform to the anatomy of the patient""s femur.
Such a need for prostheses of varying shapes and sizes this creates a number of problems in regard to use of a one-piece prosthesis. For example, a hospital or surgery center must maintain a relatively large inventory of prostheses in order to have the requisite mix of prostheses needed for certain situations such as trauma situations and revision surgery. Moreover, since the bow of the stem must conform to the bow of the intramedullary canal of the patient""s femur, rotational positioning of the upper portion (i.e. proximal end) of the prosthesis is limited thereby rendering precise locating of the upper portion and hence the head of the prosthesis very difficult. In addition, since corresponding bones of the left and right side of a patient""s anatomy (e.g. left and right femur) may bow in opposite directions, it is necessary to produce xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d variations of the prosthesis in order to provide anteversion of the bowed stem thereby further increasing the inventory of prostheses which must be maintained.
As a result of these and other drawbacks, a number of modular prostheses have been designed. As its name implies, a modular prosthesis is constructed in modular form so that the individual elements or features of the prosthesis can be selected to fit the needs of a given patient""s anatomy. For example, modular prosthesis have been designed which include a proximal neck component which can be assembled to any one of numerous distal stem components in order to create an assembly which fits the needs of a given patient""s anatomy. Such a design allows the distal stem component to be selected and thereafter implanted in the patient""s bone in a position which conforms to the patient""s anatomy while also allowing for a limited degree of independent positioning of the proximal neck component relative to the patient""s pelvis.
One issue that arises as a result of use of a modular prosthesis is the locking of the components relative to one another. In particular, firm locking of the proximal neck component to the distal stem component is critical to prevent separation of the two components subsequent to implantation thereof into the patient. As such, a number of locking mechanisms have heretofore been designed to lock the components of a modular prosthesis to one another. For example, a number of modular prostheses have heretofore been designed to include a distal stem component which has an upwardly extending post which is received into a bore defined in the distal neck component. A relatively long fastener, such as a screw or bolt, is utilized to secure the post within the bore.
However, such a design has a number of drawbacks associated therewith. Firstly, functional loading during use of the prosthesis may not provide a positive lock and may actually tend to urge the upwardly extending post of the distal stem component out of the bore defined in the proximal neck component. In such a case, the fastener (e.g. the screw or bolt) alone must absorb such loads. This creates a number of problems since many of such functional loads tend to be axial in nature. In particular, by the nature of its design, axial loads exerted on a fastener such as a screw or bolt bear on the threads of the fastener thereby undesirably exerting a relatively large load to a relatively small surface area. Over time, such loads may degrade or even breach the mechanical integrity of the threads thereby potentially allowing the components to separate from one another.
Secondly, manufacture of such modular prosthesis is relatively difficult and, as a result, expensive. In particular, in order to utilize a long screw or bolt to secure the two components to one another, a relatively long bore must be drilled or otherwise machined through the entire length of the proximal neck component and at least a portion of the length of the distal stem component. Such drilling, often referred to as xe2x80x9cgun drillingxe2x80x9d, is relatively difficult to do since, amongst other things, it requires adherence to extremely strict tolerances thereby increasing costs associated with manufacture of the modular prosthesis.
What is needed therefore is a modular prosthesis which overcomes one or more of the above-mentioned drawbacks. What is particularly needed is a modular prosthesis which has enhanced locking characteristics relative to heretofore designed modular prostheses. What is further particularly needed is a modular prosthesis that is xe2x80x9cself-lockedxe2x80x9d by the functional loads generated during use of the prosthesis.
In accordance with one embodiment of the present invention, there is provided a modular prosthesis. The modular prosthesis includes a first component having defined therein an elongated bore which has a minimum diameter which defines a first inner diameter, a first threaded aperture, and a counterbored cavity which has a second inner diameter. The elongated bore, the first threaded aperture, and the counterbored cavity are positioned coaxially with one another. The first threaded aperture is interposed between the elongated bore and the counterbored cavity. The modular prosthesis also includes a second component having a post with a number of threads extending therefrom. The number of threads have an outer diameter. The number of threads threadingly engage the first threaded aperture when the number of threads are advanced therethrough. The outer diameter is smaller than both the first inner diameter and the second inner diameter.
In accordance with another embodiment of the present invention, there is provided a method of performing a joint replacement procedure by use of a modular prosthesis. The modular prosthesis includes a first component having defined therein an elongated bore, a first threaded aperture, and a counterbored cavity having an inner diameter. The modular prosthesis also includes a second component which has a post which includes an extension portion having a first number of threads extending therefrom. The first number of threads have an outer diameter which is smaller than the inner diameter. The method includes the step of advancing the extension portion of the post through the elongated bore. The method also includes the step of threadingly advancing the first number of threads into a first end of the first threaded aperture subsequent to the step of advancing the extension portion of the post through the elongated bore. The method further includes the step of advancing the first number of threads out of a second end of the first threaded aperture and into the counterbored cavity. Moreover, the method includes the step of implanting the first component and the second component into a bone of a patient.
In accordance with a further embodiment of the present invention, there is provided a modular femoral prosthesis. The femoral prosthesis includes a stem member adapted to be implanted into a medullary canal of a femur. The stem member has defined therein an elongated bore having a minimum diameter which defines a first inner diameter, a first threaded aperture, and a counterbored cavity having a second inner diameter. The elongated bore, the first threaded aperture, and the counterbored cavity are positioned coaxially with one another. The first threaded aperture is interposed between the elongated bore and the counterbored cavity. The femoral prosthesis also includes a neck member having a post with a number of threads extending therefrom. The number of threads has an outer diameter. The number of threads threadingly engage the first threaded aperture when the number of threads are advanced therethrough. The outer diameter is less than both the first inner diameter and the second inner diameter.
In accordance with yet another embodiment of the present invention, there is provided a modular prosthesis. The modular prosthesis includes a first component having a sidewall. The sidewall has defined therein an elongated bore, a first threaded aperture, and a counterbored cavity. The elongated bore, the first threaded aperture, and the counterbored cavity are positioned coaxially with one another. The first threaded aperture is interposed between the elongated bore and the counterbored cavity. The modular prosthesis also includes a second component having a post with a number of threads extending therefrom. The number of threads threadingly engage the first threaded aperture when the number of threads are advanced therethrough. Each of the number of threads is spaced apart from the sidewall when the number of threads are positioned in the counterbored cavity.
It is therefore an object of the present invention to provide a new and useful modular prosthesis.
It is moreover an object of the present invention to provide an improved modular prosthesis.
It is a further object of the present invention to provide a new and useful method of performing a joint replacement procedure by use of a modular prosthesis.
It is also an object of the present invention to provide an improved method of performing a joint replacement procedure by use of a modular prosthesis.
It is yet another object of the present invention to provide a modular prosthesis which has enhanced locking characteristics relative to heretofore designed modular prostheses.
It is moreover an object of the present invention to provide a modular prosthesis that is xe2x80x9cself-lockedxe2x80x9d by the functional loads generated during use of the prosthesis.
It is also an object of the present invention to provide a modular prosthesis that provides a high degree of flexibility in regard to the positioning of the head component thereof relative to the patient""s acetabulum.
The above and other objects, features, and advantages of the present invention will become apparent from the following description and the attached drawings.